1. Field of the Invention
The present invention relates generally to the art of pressure sensing elements, and more specifically to the use of modern engineering plastics for certain components of such devices. Still more specifically, the present invention relates to such devices where seals are hermetically formed between the several liquid crystal polymer components and between certain liquid crystal polymer components and certain metal components and/or certain polyimide film components, the end product being useful in a wide variety of environments and for sensing a wide variety of pressures, e.g. from 20 inches of vacuum to more than 500 psig. In its most specific form, the present invention also employs a polyimide, single convolution diaphragm.
2. Description of the Prior Art
Pressure controls for a variety of applications have been made for a number of years by the assignee of the present invention. The types of controls with which the present invention is concerned have been sold under the PENN trademark and include pressure controls for refrigeration and air conditioning equipment. In such devices a tube is connected to the environment to be measured and to a control housing through a fitting known to the trade as a controller "cup". Within the cup, a diaphragm is spring biased and mounted in such a way that a bushing moves in response to the pressure of the fluid or gas contained within the environment. Such movement can be simply read or used by the controller to carry out certain control functions, which in and of themselves are known and form no part of the invention.
As illustrated in the three product bulletins supplied with this application (3496-E, 3848-A and 3864B), the controllers may be variously configured in single pole or double pole arrangements, and they are designed for use with different fluids (e.g. known refrigerants such as R-12, R-22, R-500, R-502 and R-134A) and for a variety of pressure levels (e.g. from 20 inches of vacuum to more than 475 psig). These controls are designed for use as operating controls where a small pressure differential may be detected, e.g. 5 psi, typically direct controlled to AC motors with numerous setting choices, either CLOSE High - OPEN Low or OPEN High - CLOSE Low.
While the controls described above and in the accompanying literature function very efficiently, they do have several drawbacks, especially in the nature of their materials of construction and methods of manufacture. Presently most, if not all commercial pressure vessels used for controllers as well as other devices, utilize steel stampings, turned steel or brass, iron or aluminum castings and welding, brazing, soldering, screwing or crimping processes are employed for securing fittings, brackets or tubes to the pressure vessel. These materials and processes are very expensive and currently result in significant scrap.
In addition to the combining of such parts in a device, such as the aforementioned controllers, it is necessary to couple the pressure vessel (in this case the cup) to the tubing by a process which will result in a hermetic seal or joint. Currently this is accomplished via pinching a relatively high modulus material such as steel or brass against a softer material, such as rubber or plastic, or by using a third material such as an adhesive, O-ring or braze mat between two high modulus materials. O-rings are the preferred choice because the relatively inexpensive part can be loaded to give acceptable results. 0-rings are not suitable where the temperatures or clamping forces do not permit their use, in which case an adhesive may be used. However, curing problems may be encountered with adhesives, or they may not be acceptable in certain harsh environments where chemical or thermal instability may cause premature failure.
Brazed joints are expensive per piece unless large furnaces are available. The latter obviously increases operating costs. Laser welding also provides a good joint, but expensive equipment is necessary and the process is slow and requires extremely clean parts. Soldering uses expensive material and is considered to be quite slow. Tig welding is cheaper, but is more typically used for securing than sealing. Lap welding automation has been slow to develop and suffers from generally slow through-put and unacceptably high joint temperatures. These joint costs are normally justified because of the high reliability required. Joint integrity is especially important in the refrigeration industry, where inoperability and CFC leakage are both substantial concerns. Moreover, heretofore brazing, welding and soldering have been viewed as the only techniques which are durable enough to survive both the harsh chemical environment and the abusive vibration conditions which exist in refrigeration equipment. Testing by agencies, such as Underwriter's Laboratory, also has dictated the use of mechanical type joints in most past systems.
A need therefore exists for pressure control devices which can be prepared more quickly and just as reliably as those currently on the market. Material and labor cost savings would both be highly desirable.
In fields other than those with which the present invention is concerned, new engineering plastics have been developed for a variety of end use applications. One type of engineering plastic is the "LCP" type, i.e. liquid crystal polymers. These resins will be discussed in greater detail below in connection with a description of the preferred embodiment, but generally they are known as highly resistant materials which exhibit toughness, dimensional stability, flame resistance and ease of processing. They are inert to such chemicals as acids, bases, aromatic and halogenated hydrocarbons, etc. They also resist stress cracking in the presence of most chemicals at elevated temperatures and pressures.
One manufacturer of LCP is Hoechst Celanese Corporation which markets a number of such resins under the VECTRA.RTM. trademark. That company describes its LCP resins as wholly aromatic polyesters useful for injection molding and extrusion opportunities, especially when reinforced with fibers, fillers and the like. Principal physical characteristics are said to be V-O ratings down to 1/64", high strength and modulus, chemical resistance (even under stress), controllable, very low co-efficient of linear thermal expansion, high dielectric strength and high heat deflection temperature. Molding advantages are said to be fast cycle times, low shrinkage, low melt viscosity, resistance to warpage, etc. Usage temperatures are rated between -325.degree. F. to +460.degree. F. VECTRA.RTM. polymers are recommended by their manufacturer for electronic applications, relays, housings, switches, fiber optic strength members, automotive components, motor components, shafts, housings, microwave equipment, watch components, etc. Four technical brochures from Hoechst Celanese Corporation are supplied herewith (TVC 7B/5M-88, VC-7 and VC-3 and VC-4).
Several applications for LCP type resins have also been disclosed in the patent literature. See, for example, U.S. Pat. No. 4,825,166 issued Apr. 25, 1989 to MacGugan for "Bobbin For A Magnetic Sensor". In this patent, the plastic frame for carrying a magnetic core is made from poly-ether-ether-ketones, an LCP class material Another use is shown in U.S. Pat. No. 4,880,591 issued Nov. 14, 1989 to Baba for "Method For Manufacturing Speaker Vibration Member". In this patent, at least ten percent (10%) of a thermoplastic LCP with 3-40% fiber or flake fill is used as the main structural material for a sound speaker.
An LCP is used as a molded thermoplastic substrate in a "Pressure Transducer With Conductive Polymer Bridge" which is the subject of U.S. Pat. No. 4,852,581 issued Aug. 1, 1989 to Frank. Conductive polymers provide the resistive elements of a Wheatstone bridge in this device.
A "High Voltage Contact Assembly" is described in U.S. Pat. No. 4,834,678 issued May 30, 1989 to Emadi, et al. Housing sub-assemblies are ultrasonically welded in this device which may be made from LCP materials.
An "Electrical Inductive Apparatus" is described in U.S. Pat. No. 4,599,594 issued Jul. 8, 1986 to Simon. The winding tube of this device may be made from LCP class resins, and sub-assemblies thereof may be permanently joined by ultrasonic welding.
Several examples of patents which describe LCP resins themselves include U.S. Pat. No. 4,871,817 issued Oct. 3, 1989 to Rock and entitled "Polyetherimide-Liquid Crystal Polymer Blends", U.S Pat. No. 4,260,695 issued Apr. 7, 1981 to Medem, et al. for "Process For The Preparation 0f Aromatic Polyesters" and U.S. Pat. No. 4,861,857 issued Aug. 29, 1989 to Kricheldorf, et al. and entitled "Thermotropic Polyesterimide From N-Carboxyalkylene Trimellitimide Having Excellent Processibility And Process For Their Production And Their Use For The Production Of Moldings, Filaments, Fibers And Films".
A Hoechst Celanese Corporation patent describes a bonding processing using Ph adjustment (U.S. Pat. No. 4,863,787 issued Sep. 5, 1989 to Gerg, et al.), and a "High Temperature Fluid Sensor" is shown in U.S. Pat. No. 4,881,055 issued to Capp, et al. on Nov. 14, 1989. In the latter, an LCP is used as a housing material for electrical conductors which project from opposite ends of the body and which are shrouded with and electrically connected by fusible material that has been molded in place. Nowhere are LCP's suggested for use as a pressure vessel, either in existing literature or patents.